Tonometers for measuring IOP (intraocular pressure) were originally developed as “contact” type instruments, meaning that a portion of the instrument is brought into contact with the cornea during the measurement procedure. A well-known instrument of this type is the Goldmann applanation tonometer originally developed during the 1950s. The GAT measures the force required to flatten (“applanate”) a known area of the cornea, and is used today as a standard against which other types of tonometers are calibrated and compared to assess measurement accuracy.
Patient discomfort and the requirement to use anesthesia related to contact tonometers such as the GAT led to the development of “non-contact” tonometers (NCTs) which operate by directing an air pulse at the cornea to cause applanation. Typically, the air pulse is generated by a solenoid driven pump mechanism and directed through a narrow fluid discharge tube at the cornea. As the cornea is deformed by the fluid pulse, an opto-electronic system monitors the cornea by detecting corneally reflected light from a beam incident upon the cornea, and a peak detector signal occurs at the moment of applanation when the reflecting surface of the cornea is flat.
In state of the art NCTs, a pressure transducer detects a plenum pressure in the pump mechanism as the pulse is generated and provides a plenum pressure signal proportional to the plenum pressure. The plenum pressure signal and applanation signal are processed to determine the plenum pressure at the moment of applanation. The plenum pressure at applanation is converted to an IOP value in units of mmHg (millimeters mercury) using a regression equation developed and stored in instrument memory during clinical calibration relative to GAT as a reference. A primary index of an NCT's reliability is the standard deviation of differences Sd of matched pairs of NCT and GAT clinical readings.
While NCTs provide reasonably reliable IOP measurements, IOP readings are occasionally falsely inflated because some of the air pulse energy is expended “bending” the corneal tissue itself, as opposed to displacing intraocular fluid pressing on the cornea. Intuitively, a cornea that is very rigid is more likely to cause a falsely elevated pressure reading because more air pulse energy is required to achieve applanation. In fact, several recent studies indicate that physical properties of the cornea can have a significant impact on NCT readings. See, for example, Copt R-P, Tomas R, Mermoud A, Corneal Thickness in Ocular Hypertension, Primary Open-Angle Glaucoma, and Normal Tension Glaucoma, Arch Ophthalmol. Vol. 117:14-16 (1999); Emara B, Probst L E, Tingey D P, Kennedy D W, et al., Correlation of Intraocular Pressure and Central Corneal Thickness in Normal Myopic Eyes After Laser in situ Keratomileusis; J Cataract Refract Surg, Vol. 24:1320-25 (1998); Stodtmeister R, Application Tonometry and Correction According to Corneal Thickness, Acta Ophthalmol Scand, Vol. 76:319-24 (1998); and Argus W A, Ocular Hypertension and Central Corneal Thickness, Ophthalmol, Vol. 102:1810-12 (1995). For persons with relatively thick corneas, IOP values measured under prior art methodology can differ significantly from “true” IOP. Heretofore, attempts to correct measured IOP for corneal thickness effects have typically involved measuring corneal thickness by additional instrument means and correcting measured IOP by an amount based upon the measured corneal thickness. U.S. Pat. No. 5,474,066 issued Dec. 12, 1995 to Grolman ascribes to this approach.
A weakness with respect to corrections based on corneal thickness is that corneal thickness is a static parameter that may or may not be a reliable indicator of a cornea's rigidity in response to dynamic loading by an air pulse or other means of applying force to cause applanation. Stated differently, corneas having the same thickness may exhibit different rigidity responses under static or dynamic loading due to differences in the corneal tissue itself. The present applicant, in his U.S. patent application Ser. No. 09/553,111, now U.S. Pat. No. 6,419,631, describes a non-contact tonometry method wherein two plenum pressures are taken into account for correlation to IOP, the first corresponding to an applanation state of the cornea upon inward deformation by an air pulse and the second corresponding to an applanation state of the cornea as it returns from a brief concave state to its normal convex state. In accordance with the described method, it is assumed that corneal rigidity force components associated with inward and outward deformation essentially cancel each other out, and the IOP measurement value is taken either by correlating the inward and outward plenum pressures to IOP based on two separate regression equations and averaging the resultant pair of IOP values, or by averaging the inward and outward plenum pressures and correlating the average pressure to IOP using a single regression equation. While this method is an improvement over the prior art, it is based on an observance of the second applanation event, which is an accidental by-product of excess impulse energy being delivered to the eye beyond the threshold level necessary to achieve the first applanation event. This excess energy is largely considered undesirable by those skilled in the art because it causes patient discomfort during testing. Consequently, developers of non-contact tonometers have sought to minimize excess impulse energy, for example by shutting off or reversing the pump driver at or before the first applanation event, building a pressure release valve or the like into the pump system, and by altering the shape of the pressure ramp itself. In this regard, please see U.S. Pat. Nos. 5,779,633; 5,165,408; and 6,159,148.
Thus, the in/out tonometry method described above suffers in certain respects. The method itself relies on dissipation of the fluid pulse in an uncontrolled manner, such that the plenum pressure as a function of time forms an asymmetrical curve about a peak pressure associated with the pump compression stroke. This fact to some extent undermines the basic assumption of force cancellation in the dynamic system. Also, the use of a non-contact tonometry method that requires delivery of excess impulse energy to the eye is largely incompatible with non-contact tonometers designed to reduce air puff discomfort felt by the patient, and may be unnecessary in situations where the patient's IOP is well within a normal range.